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Proper Fertilization of Home Lawns

Proper fertilization leads to healthy lawns (Figure 1) and reduces the chance of fertilizers reaching our groundwater. Research has shown a properly fertilized lawn actually reduces the amount of nitrate in the groundwater because the extensive root system developed as a result of proper fertilization utilizes the nutrients more efficiently and allows less to leach through the soil. In addition, a strong root system can make the lawn more resistant to summer drought.

Healthy lawn

Figure 1 - Healthy lawn

There are many brands and formulations of lawn fertilizers. A basic understanding of what the numbers on the bag mean will help you decide which type of fertilizer is best for your lawn. Fertilizers are identified by their analysis, that is, the three numbers on the bag. The numbers refer to the percentage of nitrogen, phosphorus (expressed as P2O5), and potassium (expressed as K2O) in the fertilizer. A 100 lb. bag of 29-4-8 contains 29 percent nitrogen, 4 percent phosphate, and 8 percent potash, or converted to weight, 29 lbs. nitrogen, 4 lbs. phosphate, and 8 lbs. potash. The rest is inert ingredients, included to help distribute the nutrients evenly.

Nitrogen can really "green up" a lawn. However, some forms of it are very mobile and can leach through the soil to contaminate the water supply if not properly applied. The nitrate form of nitrogen (NO3) can move with the soil water and contribute to pollution and algae blooms; it is of greatest concern in water quality. The positively charged ammonium form of nitrogen (NH4) can attach to the negatively charged soil particles and remain in the soil, but oxidizes to the nitrate form rapidly. There is a form of nitrogen, however, that will not move through the soil very fast. It's called Water Insoluble Nitrogen (WIN), and the fertilizer bag indicates what percentage of the total nitrogen is in this form. Water Insoluble Nitrogen dissolves more slowly and will stay in the plant root zone longer.

Lawns vary in their need for nitrogen. Maintaining an existing lawn requires less nitrogen than starting a new lawn. Maintenance applications should be split and applied over a few months, so there will be less danger of leaching. A healthy root system will absorb most of the leaching nutrients.

The second number on the bag is phosphate (P2O5). New lawns benefit from a high relative level of phosphorus. This helps with seed germination and gets the roots off to a good start. Phosphorus is fairly stable and is held by the soil particles. There is a chemical balance of phosphorus in the soil, with some being readily available and some held in reserve. But if erosion occurs, phosphorus can move with soil particles and contribute to pollution. Phosphorus also contributes to algae growth, although not as seriously as nitrogen.

The third ingredient on the bag, potash (K2O), is a source of potassium. It contributes to the strength of the plant and helps the plant form carbohydrates. It does not contribute much to water pollution as potassium can bind to soil particles. However, if there is an erosion problem, this nutrient can move with the soil particles. A healthy plant cover will greatly reduce soil erosion.

The purpose of a soil test for your lawn is to supply you with enough information to make a wise fertilizer choice. This can help save money because you won't be adding nutrients your soil doesn't need.

Lawns grow best at a pH of 6.2 to 6.5. That is, the nutrients are most available to the grass plants in this pH range. The soil test will give specific recommendations on how much to apply to your particular area. When applying fertilizers, avoid getting it on paved surfaces; it can easily wash into storm drains, resulting in water pollution.

In areas where cool-season grasses (Kentucky bluegrass, tall fescue, perennial rye grass, creeping red fescue) are grown, fertilizer should be applied in the fall. September, October, and November/December are best. Splitting the total yearly amount of fertilizer into three applications is recommended to minimize the potential for leaching.

The advantages of late fall fertilization are increased density, increased root growth, decreased spring mowing, improved fall to spring color, decreased weed problems, increased drought tolerance, and decreased summer disease. This will help the lawn green up faster in the spring and make it more tolerant of heat and drought stress.

Grasses, such as Bermuda grass and Zoysia grass, do best when fertilized in the spring and summer. April, May, and July applications are best. A late summer application could lead to winter injury due to excessive vegetative growth.

Your responsibility in using fertilizers safely and effectively results in a healthier lawn and better water quality.